Implement the urgency hint for windows/workspaces

Thanks to Mikael for bringing it to my mind. This change introduces
two new color classes, client.urgent and bar.urgent. By default,
urgent clients are drawn in red (colors by Atsutane).

include/config.h

@@ -64,10 +64,12 @@ struct Config {
                 struct Colortriple focused;
                 struct Colortriple focused_inactive;
                 struct Colortriple unfocused;
+                struct Colortriple urgent;
         } client;
         struct config_bar {
                 struct Colortriple focused;
                 struct Colortriple unfocused;
+                struct Colortriple urgent;
         } bar;
 };